1. Technical Field
The present invention relates to an oxygen ion conductor and a solid fuel cell.
2. Technical Background
In recent years there has been increasing interest in solid oxide fuel cells in which oxygen ion conductor is used as an electrolyte. In particular, from the aspect of effective energy utilization, solid fuel cells have such excellent advantages as having an intrinsic high energy conversion efficiency which is not restricted by Carnot's efficiency, and being environmentally safe.
Conventionally, oxygen ion conductors have held the greatest degree of promise as the electrolyte for use in a solid oxide fuel cell. However, in order to obtain a sufficient degree of ion conductivity in Y.sub.2 O.sub.3 stabilized ZrO.sub.2 (yttriia stabilized zirconia: YSZ), which is one type of conventionally known oxygen ion conductor, high temperature operations of 1000.degree. C. are necessary.
In other words, in a construction wherein an oxygen electrode, a solid electrolyte and a fuel electrode are laminated, when an oxygen gas concentration gradient is applied at both sides of the solid electrolyte, the oxygen ions diffuse throughout the solid electrolyte and a fuel cell is formed by means of an electrochemical reaction at the interface of the electrodes. When solid oxide fuel cells are operated at 1000.degree. C., reactions arise at the interface between the solid electrolyte and the fuel electrolyte, causing deterioration.
Namely, because the deterioration in the life of the components due to reactions between the electrode and electrolyte becomes severe at high temperatures such as noted above, the practical application of solid fuel cells has been delayed. Because, from this point of view, it is desireable to lower the operation temperature, the realization of an ion conductor material having a degree of ion conductivity which is higher than that of YSZ is desired.
In general, in a zirconia system oxygen ion conductor, the ion conductivity tends to increase as the ionic radius of the dopant becomes smaller. This is because, as the size of the ionic radius of the dopant approaches the size of the ionic radius of Zr.sup.4+, the activation energy of the mobile oxygen ion becomes smaller. In fact, it is known that, of zirconium system oxygen ion conductors, a ZrO.sub.2 --Sc.sub.2 O.sub.3 system oxygen ion conductor shows the highest degree of ion conductivity.
However, accompanying an increase in the dopant, the crystal structure of the oxygen ion conductor changes from monoclinic to rhombohedral to cubic. Further, there is an additional problem in that, within the region in which a maximum value can be attained for ion conductivity, the rhombohedral crystal structure becomes stable at room temperature, while the cubic crystal structure is not stable. Moreover, at temperatures above 650.degree. C., when a heat cycle is provided such that temperature conditions exceed 650.degree. C. in order to accomplish structural phase transition to a cubic crystal structure, fracturing is brought about easily. Thus, practical application as a solid electrolyte material is not possible.